1. Technical Field
The present invention relates generally to semiconductors and more particularly to a method of inspecting fill patterns on semiconductor masks.
2. Prior Art
A semiconductor is a material having a value of resistivity midway between that of conductors and insulators. Although a raw semiconductor material cannot transfer electricity or light, when heat, light, or impurities are added, electricity can be transferred. Moreover, the amount of the transfer can be controlled. Semiconductor manufacturing is generally divided into three phases: fabrication, assembly, and testing. During the fabrication stage, a wafer of raw semiconductor material is typically treated through a process referred to as photolithography in preparation for the fabrication of integrated circuits.
Photolithography process generally involves exposing light through a photomask onto a photoresist coating on a wafer. Photomask typically includes a transparent portion and an opaque portion, thereby forming a pattern and selectively exposing the light to the photoresist coating according to the pattern. Photoresist coatings are produced from organic solutions which, when exposed to light of the proper wavelength, are chemically changed in their solubility to certain solvents (developer). For example negative-acting resist is initially a mixture which is soluble in its developer but after light exposure becomes polymerized and insoluble to the developer. Thus, when the light is exposed through a photomask having a pattern formed by its transparent and opaque portions onto the photoresist, the unexposed resist selectively dissolves, softens, or washes away, leaving the desired pattern on the underlying wafer. Positive-acting resists work in the opposite fashion, i.e., exposure to light makes the polymer mixture soluble in the developer. Thus, in positive-acting resists, the transparent portions of the mask correspond to the desired pattern or image to be provided in the resist coating. The remaining pattern then is treated through the rest of the fabrication process for forming a desired semiconductor device.
The patterns formed on the semiconductor provides the yield and reliability of the semiconductor devices. Therefore, it is highly desirable that such patterns have a high degree of accuracy. Moreover, because the quality of the pattern formation is determined largely by the quality of the photomask, it is also highly desirable that the photomasks have an accurate and precise patterns.
Presently, as the complexity and amount of circuitry within each individual integrated circuit increases, there is an increase in the number of layers used in a single chip, affecting the planarity and uniformity of topology underlying some layers. The planarity or the uniformity of topology underlying a deposited layer of material can have a significant impact on the ability to pattern and etch the deposited layer. Therefore, for masks used on some lithography levels or layers a fill is added, typically by a mask manufacturer, to assure constant loading of a pattern. A fill generally refers to a series of contact holes.
The masks are then tested to ensure that the desired accuracy and preciseness is achieved in semiconductor manufacturing by employing various inspection tools utilized to inspect and detect defects in pattern formations. Most of the known inspection tools generally compare images to design and/or images to images to detect malformations in the patterns. The fill images physically present on a mask generally have no critical function in the printed image and need not undergo an equally intense inspection phase. During the testing phase, however, these additional fill pattern images create additional challenges, unnecessary work, and detractions. For instance, the fill patterns are inspected as a main pattern even if it is not necessary to achieve proper loading. Since operators cannot distinguish between real pattern and fill defects, the masks are inspected using the same criteria as a functional pattern, thereby creating significant additional defect counts and workload. Because there is an uncertainty whether or not a pattern is important, all defects are repaired during the inspection process, creating unnecessary handling, inspections and repairs which in some instances may lead to plate scraping and damages. Therefore, it is highly desirable to be able to identify the fill patterns which need not pass an inspection process, and to be able to omit unnecessary inspections and repairs of these fill patterns.
The present invention is a system implemented for marking the fill images so that an operator or an automatic software system can easily recognize and avoid unnecessary inspection and repairs in the fill area. The fill images are made in an unusual shape typically not occurring on a plate. Such shapes may include serifs in the middle of the side of the contact, rather than the usual corner serifs. The shapes may also have alphanumeric forms.
In another embodiment, subresolution markers having values, for example, of 0.01 micrometers (um) may be placed to designate fill images and or regions on a design plate image data that are used by inspection tools for inspecting plates. The inspection software or tool in conjunction with the markers may be programmed to sort the data and create xe2x80x9cdo not inspectxe2x80x9d or reduced sensitivity regions for the areas where the markers are placed. The inspection software or tool would then run at full sensitivity only on the real patterns, skipping the inspection or inspecting at a reduced sensitivity of the marked areas which represent the fill images.
Identifying large areas in which fill shapes are placed and programming an inspection tool to either to skip these areas or to inspect with a larger granularity of sensitivity, for example, to inspect defects only of 0.5 micron dimensions or greater would result in a highly efficient mask testing process.
Advantageously, the fill areas are typically defined by a circuit designer on a separate layer of data so that the locations of fill areas are easily identified and quantified. When the locations of the fill areas are identified from the separate layer having fill patterns, the subresolution markers may then be placed in the areas in the pattern image corresponding to the identified areas.